1. Field of the Invention
The present invention relates to multilayer printed wiring boards, and more particularly to a method and apparatus for accurately registering laminated printed wiring boards for drilling operations.
2. Description of the Prior Art
Multilayer laminated printed circuit wiring boards are commonly used in complex, high density electronic circuits. The individual layers of a laminated board may be composed of glass-fabric reinforced substrates for a rigid board, and of unreinforced film for a flexible board. Each substrate board may have a circuit wiring pattern of copper foil on one or both sides thereof. Each conductor terminates in a terminal area or pad. The wiring pattern is formed by etching of a sheet of copper foil bonded to the substrate board prior to lamination.
The outer faces of the outermost substrate layers have unetched copper foil bonded thereto either prior to or at the time of lamination. One or more insulating bonding sheets consisting of an adhesive matrix impregnated into a glass fabric substrate or an insulating bonding film having an adhesive medium on the surfaces thereof are disposed between each pair of inner substrate boards. The multiple layers of boards are laminated by applying heat and pressure to form a relatively thick composite panel. A typical panel thickness may be 0.062 inches. As will be recognized, a composite panel is thus produced having no distinguishable external features.
It is necessary to now interconnect terminal areas of the inner substrate boards with the outside surface printed wiring by first drilling through-holes which should pass through the centers of the generally circular terminal pads of the substrates. The drilling is most commonly accomplished by means of a numerically controlled (NC) drilling machine, digitally programmed with the coordinates of each point on the panel to be drilled as specified by the design data. After drilling through the terminal areas, the entire panel is electroplated, plating the outer unetched copper foil as well as producing a conductive cylinder through each through-hole. The cylinder bonds to the edges of any printed wiring pad in each layer that occurs at that through-hole location.
However, the processes of etching the substrate board surfaces to produce the wiring, and the heat and pressures of laminating, cause the internal terminal areas in the laminated panel to be shifted slightly from their design locations. Such shifting is due to several factors, including: release of stresses which were formed when the copper foil is bonded to the insulation layers, then relieved during etching away of the nonfunctional copper; release of stresses during a baking operation after etching and cleaning; and shifting of terminal areas during the final lamination process. The shifting from these causes is unpredictable and must be specifically determined for each panel.
As will now be understood, each terminal area will be shifted from its design center x,y location to an unknown center x',y' location. If holes were drilled at the x,y locations, a hole would not pass through the actual x',y' centers and the integrity of the panel would be compromised. Although the specific locations of the centers are not known, it has been found that use of individual substrate layers composed of the same materials and construction, and processed in the same manner will have the magnitude and directions of dislocation to be approximately equal. Thus, if the center location of a terminal area is known for one layer, the corresponding point on the other layers will be known within acceptable limits.
A common prior art method for solving this problem is to drill a pilot hole somewhat smaller than the through-hole design specification using a drill machine programmed to place the hole in a target terminal area at a center specified by the design data. The drilled panel is removed from the drill machine and X-rayed, by fluoroscopic or radiographic means, to determine the position of the pilot hole with respect to the true center of the shifted target area. The direction and magnitude of the target area displacement is accurately measured and a drill position offset factor entered into the drill controller. The functional holes may then be correctly drilled.
This method is expensive and time consuming since the panel must be drilled, removed from the drill bed, X-rayed, an offset determined by measurement on the radiograph, and then returned to the drill bed. Thus, there is a need for a method which permits determination of center location corrections without the necessity for X-rays or removal of the panel from the drill bed.